1. Field of the Invention
The present invention generally relates to a tone detector, and more particularly, to a tone detector transmitting a tone signal while performing an accurate tone detection without using a filter or consuming a cost.
2. Description of the Related Art
In a communication apparatus, such as a facsimile apparatus, a tone signal needs to be detected upon performing a communication, such as a transmission/reception of data. Further, there is a case where, while transmitting a tone signal (hereinafter referred to as a tone T), a different tone signal (hereinafter referred to as a tone R) having a different frequency needs to be detected. In this case, an echo signal of the transmitted tone T is returned due to various causes; thus, not only the tone R, but also the echo signal of the tone T, enter a detector detecting the tone R. This makes it difficult to detect the tone R. To solve this problem, influences of the echo signal need to be reduced or eliminated by removing the echo signal.
Conventionally, in general, a filter is used for removing the echo signal of the tone T. Specifically, a filter cutting the echo signal of the tone T is provided before the detector detecting the tone R so as to remove components of the echo signal of the tone T from input signals including the echo signal of the tone T and the tone R, or to reduce a level of the echo signal.
However, when the tone T and the tone R have approximate frequencies, a high-order filter sometimes needs to be used. Without such a high-order filter, components of the echo signal of the tone T cannot be removed sufficiently; this exposes the detector detecting the tone R under influences of the echo signal. Additionally, without such a high-order filter, not only the echo signal of the tone T, but also signal components of the tone R are removed so as to reduce a level of the tone R; this imposes negative effects on the detector detecting the tone R.
Consequently, a high-order filter is used in a conventional tone detector; however, preparing a high-order filter in the form of hardware necessitates the number of components to be increased, which leads to an increased cost. On the other hand, using a high-order filter in the form of software takes a long processing time due to numerous stages of the filter, which reduces a throughput of the tone detector as a whole.
Further, in a communication apparatus, such as a facsimile apparatus, a DTMF signal and various types of tone signals need to be detected. In one case, one type of tone signal (hereinafter referred to as single tone) is detected; in another case, two types of tone signals (hereinafter referred to as dual tone) are detected simultaneously.
Conventionally, these tone signals are detected by detectors specialized for individual usage, such as a detector used for the DTMF signal, a detector used for the single tone, and a detector used for the dual tone.
However, since the above-mentioned conventional detectors are specialized for individual usage, preparing the conventional detectors in the form of hardware takes a high cost, or leads to an increase in the number of components. Alternatively, preparing the conventional detectors in the form of software necessitates different processes to be provided for each of the detectors, which leads to a long processing time; accordingly, processors, such as a CPU, cannot spare computing powers for processes necessary for other communications, suffering lack of powers.